RICE UNIVERSITY STRUCTURAL GEOLOGY OF THE GASS PEAK AREA LAS VEGAS RANGE, NEVADA by WILLIAM JAMES EBANKS, JR. A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS 3 1272 00020 4303 Thesis Director's Signature Houston, Tex.z (May, 1965) Abstract A field-mapping study of the area around Gass Peak, Las Vegas Range, Clark County, Nevada, has demonstrated the presence of a thick sequence of Paleozoic and Precambrian rocks overthrust on younger, Upper Paleozoic beds along the Gass Peak Thrust. Formations recognized in the upper plate of the thrust are the Precambrian Stirling Quartzite, the Precambrian and Cambrian Wood Canyon Formation, the Cambrian, Carrara, Bonanza King, and Nopah Formations, and the Ordovician lowermost Pogonip Group. Rocks in the lower plate are the Devonian Sultan Limestone, Mississippian Monte Cristo Lime¬ stone, and the Permo-Carboniferous Bird Spring Formation. The Miocene Horse Spring Formation unconformably overlies the Paleozoic rocks. Faulting on the Gass Peak Thrust resulted in approxi¬ mately 18,000 feet of stratigraphic displacement from west- to-east. Several large folds and many high-angle reverse faults are associated with the thrust fault. All of these features, the major thrust and smaller re¬ lated structures, have been rotated westward through 90 de¬ grees by right-lateral strike-slip movement on the adjacent Las Vegas Valley Shear Zone. The "drag" structure and a unique set of faults in the Gass Peak area are related to east-west extension and north-south compression caused by i the rotation. i Strata above and below the Gass Peak Thrust are similar to strata in the Wheeler Pass Thrust, suggesting the two thrusts are offset equivalents. This implies more than 25 miles of relative horizontal shift between the two faults. Many details of structure are dissimilar between the Wheeler Pass and Gass Peak Thrusts, and if they were once the same feature, they have developed independently after being separ¬ ated. Structural evidence indicated that displacement on the Gass Peak Thrust occurred before movement along the Las Vegas Valley Shear Zone. There is little evidence in the mapped area for Tertiary block faulting. TABLE OF CONTENTS page no# I, INTRODUCTION . 1 A. PURPOSE OF INVESTIGATION 1 B. ACKNOWLEDGEMENTS ...... „. 1 C o PREVIOUS WORK oe..,oo,a.oo,.*«o,ooooa 2 II, LOCATION AND DESCRIPTION OF THE STUDY AREA 3 III, STRATIGRAPHY •,»oo»,,o«o*«,,oaeo, 3 A. METHODS ........... 5 B. GENERAL 5 C. FORMATIONS PRESENT .,... 7 1, Precambrian rocks 7 a. Stirling Quartzite 7 2• Cambrian rocks 8 a. Wood Canyon Formation 8 b. Carrara Formation .. 9 c. Bonanza King Formation •. 0 •.... 10 d. Nopah Formation •. 11 3, Ordovician rocks ................, 13 a, Pogonip Group 13 4, Devonian-Mississippian rocks (undifferentiated) 14 a, Monte Cristo Limestone and Sultan Limestone ...... 14 5, Mississippian-Pennsylvanian- Permian rocks ..o.. 14 a. Bird Spring Formation 14 6, Tertiary rocks 16 a. Horse Spring Formation ... 16 7, Quaternary rocks •»••«,«.. 19 a. Older and younger alluvium ,,,, 19 IV, STRUCTURAL GEOLOGY ,««o,oo,,20 A, REGIONAL SETTING 20 B. STRUCTURAL FEATURES OF THE GASS PEAK AREA ,e,0o,,,,,ea,e,,,,o,,,a*oeooooo, 21 U Gass Peak Thrust 21 a. General ,., 21 b. Stratigraphic displacement 22 c. Geometry o,*«,23 ii d. Footwall 23 e. Hanging wall 24 f. Decollement faulting 26 g. Other features 27 2» Structures related to thrusting • • 30 a. General .... ...... 30 b. Footwall features ... 30 Cl) Broad folds 30 (2) Reverse faults ...a........ 30 (3) Overturned folds 31 (4) Normal faults 33 c. Hanging wall features 37 (1) High-angle reverse faults • 37 (2) Broad folds 38 3. Structures related to the Las Vegas Valley Shear Zone 38 a. General ... 38 b. Extension faults 39 c. Back thrusts 41 4. Normal faulting 43 5. Late Tertiary deformation ...»..*• 44 6. Gravity sliding 44 C. TIMES OF MAJOR DEFORMATIONS 46 1. Thrust faulting 46 2. Strike-slip faulting 47 V. SUMMARY 48 VI. CONCLUSIONS 50 VII. BIBLIOGRAPHY 53 LIST OF ILLUSTRATIONS Figure 1 Index map of U.S.G.S. Quadrangles in the area of Las Vegas, Nevada . 4 Figure 2 Stratigraphic units of the Gass Peak area, Nevada «...o.• ••«..«..«......o.. 6 Figure 3 Major structural features near the • Gass Peak area, Nevada 18 Figure 4 Drag structures .......o«.o*.«o...... 40 Figure 5 Small-scale folding in the Bird Spring Formation ...................e. 25/ r 1 iii Figure 6 Overturned beds south of Dry Wash ... 32 Figure 7 Overturned beds south of Broken Hill 34 Figure 8 Normal fault dipping southward 35 Figure 9 Low-angle normal faults dipping northward 36 PLATE I Geologic Map of part of the Gass Peak Quadrangle, Clark County, Nevada •••• pocket - • I I 1. INTRODUCTION PURPOSE OF INVESTIGATION This study was undertaken to describe the structural geology in the area at the southern end of the Gass Peak Thrust, in the Las Vegas Range, Nevada. A more specific goal was the observation of structural features that might confirm or reject the postulated presence of a major strike- slip fault zone in the adjacent Las Vegas Valley. Of secondary interest, but significant to the interpretation of the geology of this complex area, was recognition of the stratigraphic units present in the mapped area. ACKNOWLEDGEMENTS / This geologist wishes to express his appreciation to the several persons whose help and criticism contributed to the completion of this thesis. First among these is Dr. B. C« Burchfiel, whose guidance in the field and super¬ vision were a valuable experience. Dr. J. Cl. De Bremaecker and Dr. R. R. Lankford also offered constructive criticism in discussions and in editing this thesis. Dr. J. L. Wilson, of Shell Development Company, kindly offered expert opinion on several Cambrian fossil identifications. The subject area is mostly within the U.S, Fish & Itfild- life Service, Desert Game Range, near Las Vegas, Nevada. The Manager of this refuge granted the writer complete freedom of access to that area. For the kind help and warm hospitality of the good people at the Corn Creek Field Station, the writer I .. is especially grateful. 2 Financial support of the writer was provided by a National Defense Education Act Fellowship. The Rice Uni¬ versity Geology Department provided field expense assistance. PREVIOUS WORK The expedition led by J. E. Spurr (1903) to explore the geology of Nevada south of the Fortieth Parallel was the first party of geologists to enter and report on the Gass Peak area. Spurr himself passed across the north end of the Las Vegas Range, but R. B. Rowe, discoverer of the Keystone Thrust, traveled the Mormon Wells Road, through the Gass Peak area, down to Las Vegas Valley. The text and map of Spurr's report indicate a slightly different topographic nomenclature than is presently used. The "Gass" or "Sheep Mountain" of that time is today called Gass Peak. Spurr's usage of "Las Vegas Range" and "New Mountains" have also been changed. Nevertheless, from Spurr's writing, it is clear that he recognized the change in strike of the formations from north- south to east-west at the south end of the Las Vegas Range. Spurr's suggestion that the "main fold of the Las Vegas Range,..., runs across the Las Vegas Valley and is probably to be found in the Spring Mountain Range on the other side...," is not acceptable in our present knowledge. Rowe's observations of stratigraphy and faulting are fairly accurate. F. C. Lincoln (1923) mentioned the Gass Peak Mining District in his early summary of mining operations of Nevada. From the most important mine, presently known as the Jung Bug Mine, zinc, gold, and silver ore was produced until abandon¬ ment in 1917. 3. C. R. Longwell, in his careful reconnaissance mapping of the one-degree Las Vegas quadrangle, covered the Gass Peak quadrangle and accurately reported the main features of the area on the map by Bowyer, Pampeyan, and Longwell (1958), Several other papers by Longwell on the Las Vegas and ad¬ jacent areas have been published (see references). Longwell (1960) first suggested the presence of the Las Vegas Valley Shear Zone and made numerous other observations and comments in the literature that have been of great assistance to this writer. Southern Nevada geology has profited greatly under his study. LOCATION AND DESCRIPTION OF THE STUDY AREA The north half of the U.S.G.S. "Gass Peak" fifteen- minute quadrangle (Figure 1) was mapped during two months of field work in the summer of 1964. Gass Peak (V.A.B.M. 6943), the dominant topographic feature of the quadrangle (Plate I), is 15 miles north of Las Vegas, Clark County, Nevada. Maximum relief encountered is about 4000 feet. Elevations range from 3000 feet to 7000 feet, but only 1000 to 2000 feet of relief is present locally. The climate is dry, desertic, with temperatures often exceeding 100°F. in the summer and precipitation averaging 4 to 6 inches, annually. Vegetation is sparse in most of the area and ecologic zonation ranges from the Creosote Bush to the transition between the Joshua Tree and Pinon- Juniper floral zones. Several fresh water springs are present 4 5 in the area, but most were dry at the time of study, and those still active were contaminated by alkali saltso STRATIGRAPHY METHODS Because the emphasis in this study is on structural geology, detailed description of the lithologic units recog¬ nized was not attempted* The writer briefly described units in the field, for comparison along strike, and hurriedly measured some of ttTe Precambrian and Lower Cambrian units, to compare thicknesses with other outcrops nearby* Dr* B* C. Burchfiel identified the various rock units in the mapped area while accompanying the writer on reconnaiss¬ ance* Some of the units are incomplete and change thickness drastically along strike, apparently because of "tectonic thinning", or replacement and omission of beds by faults within the formations* It remains to be determined whether the stratigraphic section is more complete farther north • .